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INFILTRATION by 8VG6j46

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									INFILTRATION

   AE 520




               1
       Infiltration Lesson
            Objectives
• Understand the factors affecting soil
  water infiltration
• Be able to estimate infiltration rate
  using the Horton Equation and the
  Green-Ampt Equation
• Understand the advantages of the
  various infiltration functions.

                                          2
          INFILTRATE
To cause (as a liquid) to permeate
something by penetrating its pores or
interstices.

Webster’s Ninth New Collegiate
Dictionary

                                        3
         INFILTRATION
• The movement of water through the
  soil surface into the soil profile.
• After Precipitation, it is the most
  important process controlling the
  water balance on a field.
• Expressed as total depth or as a
  velocity called the infiltration rate

                                          4
  Infiltration may be
controlled by 3 types of
         factors
  • Supply Factors

  • Surface Factors

  • Profile Factors


                           5
 Infiltration Control Supply
            Factors
• The water supply for infiltration may
  be:
• Rainfall
• Surface Pond Storage
• Irrigation
• Infiltration Rate will be the minimum
  of supply and Capacity
                                          6
 Infiltration Control Surface
            Factors
• Degree of Surface Crusting
  – Clay content of the surface
  – Effect of drop impact
• Degree of Surface Porosity
  – Soil texture
  – Soil Structure
  – Vegetation Cover
  – Inwash of fine particles
  – Cracks and other macropores
                                  7
EFFECT OF PROTECTIVE
COVER ON INFILTRATION




                        8
  Infiltration Control Profile
             Factors
• Hydraulic Conductivity of the soil
  layers
• Moisture Content of the soil layers
  near the surface.
• Rate of internal drainage



                                        9
Measurement of Infiltration
  Sprinkler Infiltrometer
         Rainfall


                               Runoff



        Infiltration

     Infiltration = Rainfall - Runoff

                                        10
      Infiltration with supply
                control


Supply rate
     Rate
        fc

                  Time
                                 11
Measurement of Infiltration
    Ring Infiltrometer




                              12
Infiltration with Soil Control
  DEPTH (F)




              TIME           13
Infiltration with Soil Control
          fo
 INFILTRATION

 DEPTH / TIME



                   dF
 RATE (f)




                f
                   dT

           fc
                 TIME        14
                             Field Surface
    Yp + Ym
              During
              Infiltration      YT

                Static
Z
                                 Soil
                                 Profile



                              Water Table
                -   0    +
                POTENTIAL
Kostiakov’s Equation

     F = CTn f = CnTn-1


                    n

                1

 C

        log T             16
            Horton’s Equation
            f  fc + f0  fc e    Kt


               Log(f0 - fc )
Log(f-fc)


                        1
                               K




                         t
                                          17
  Darcy’s Equation



     dY           dY
v K       q  KA
     dz           dz

                       18
                              SOIL MOISTURE CONTENT
                          0
DEPTH FROM INFILTRATION       ZONE OF SATURATION

                              TRANSITION   ZONE
       SURFACE



                                 TRANSMISSION
                                    ZONE




                              WETTING
                               ZONE

                                           WETTING FRONT


                                                           19
                                 HOLTAN’S CONCEPT OF


ACCUMULATED INFILTRATION DEPTH
                                     INFILTRATION




                                         TIME          20
        Holtan’s Equation
                            P
                  S F 
        f  fc + A     
                   S 


                        P
                  1
A
    1
             log ((S-F)/S)      21
Green-Ampt Equation
          KDQDY
    f K+
            F

                   KDQDY
f
               1

     K

         1/F               22
      Green-Ampt Infiltration
            Equation
       1                   Soil Surface
                Yp = 0       K = Constant
                Yz = 0       Q  Q f= constant
Wet        Lf
                Ym = Yf
                              Wetting Front
       2        Yp = 0       Q  Qi = constant
Dry             Yz = Lf
                 Ym = Yi

                                                 23
   Green-Ampt Infiltration
         Equation
dY  0  L f + Yi  0  0  Yf
 L f + Yi  Yf
FromtheDarcyEquation

V  K
       dY
           K
                L f + Yi  Yf 
       dL              Lf
                                 24
   Green-Ampt Infiltration
         Equation
• Both Yi and Yf are negative and Yi <
  Yf . Therefore DY < 0 and the
  negative signs cancel.

      DY + L f      DY 
V  K            K    + 1
      Lf            Lf    
                                         25
       Green-Ampt Infiltration
             Equation
 •   Total Depth of Infiltration = F
 •   F = Lf(Qf  Qi) = LfDQ
 •   Lf = F/DQ
 •   assuming the velocity V = the
     infiltration rate f
          DQDY      KDQDY      B
f  K 1 +       K+        A+
            F         F        F
                                       26
Green-Ampt Computations

          D YD Q     B
f  K 1 +        A+
            F        F
whe re A  K
   and B  KD YD Q
                           27
Infiltration Rate at the
 Beginning of a Time
       Increment

            B
   f1  A +
            F1

                           28
Infiltration Rate at the
 Beginning of a Time
       Increment

            B
   f2  A +
            F2

                           29
 Average Infiltration Rate
   over a Time Period
    DF   f 1 + f2
       
    DT       2
Rearranging:
      DF   f1 + f2
         -         0
      DT      2
                             30
       Upper Limit on the
Infiltration During the Period

F2  F1 1   B      B
        A + + A +   0
  DT     2  F1     F2 
   F2   F1      B   B
         A    
  DT DT        2F1 2F2

                             31
    Multiplying by F2 and
        Rearranging

 1 2           B   F1      B
   F2  ( A +    +    )F2   0
DT            2F1 DT       2
F1 will always be known or assumed.
Therefore F2 is the only unknown,
and the function is a quadratic.
                                32
 Quadratic Equation Solution
            for F2
         b  b  4ac
                 2
   F2 
              2a
     1                 B
 a=                c 
    DT                  2
          B   F1        c
b  A           A +  aF1
         2F1 DT         F1
                             33
Quadratic Equation Solution
           for F2
• Only the +sign in the quadratic
  equation applies.
• The actual value of F2 can not exceed
  F1 + Supply during the period. At the
  end of each time step, this limit must
  be checked, and if the GA equation
  gives a larger value, then the supply
  will be limiting.
                                       34
                                  Green-Ampt Infiltration
                        K = 2 mm/hr, D T = 0.25 hr, DQ = 0.1 cc/cc, DY = 5000 mm
                            Total Rainfall = 114 mm, Total Runoff = 37.26 mm
              35.0

              30.0

              25.0
Depth in mm




                                                                                Rainfall

              20.0                                                              Infilt

                                                                                Runoff

              15.0

              10.0

               5.0

               0.0
                 0.00     0.50     1.00      1.50     2.00     2.50      3.00            3.50
                                           Time in Hours

                                                                                                35
                                           Green-Ampt Infiltration
                               K = 2 mm/hr, DT = 0.25 hr, DQ= 0.1cc/cc, DY = 5000 mm
                                  Total Rainfall = 114 mm, Total Runoff = 53.15 mm
                       35

                       30
Depth in Millimeters




                       25
                                                                                    Rainfall

                       20                                                           Infilt

                                                                                    Runoff
                       15

                       10

                        5

                        0
                        0.00   0.50     1.00      1.50     2.00     2.50     3.00            3.50
                                                 Time in Hours

                                                                                                    36

								
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